Ion implantation is a process used to dope impurity ions into a semiconductor substrate to obtain desired device characteristics. An ion beam is directed from an ion source chamber toward a substrate. The depth of implantation into the substrate is based on the ion implant energy and the mass of the ions generated in the source chamber. One or more ion species may be implanted at different energy and dose levels to obtain desired device structures. In addition, the beam dose (the amount of ions implanted in the substrate) and the beam current (the uniformity of the ion beam) can be manipulated to provide a desired doping profile in the substrate. However, throughput of semiconductor devices is highly dependent on the delivered dose of the ion beam on the target substrate to produce the desired semiconductor device characteristics.
It has been found that a relatively low target substrate or wafer temperature during ion implantation reduces the required dose and consequently manufacturing throughput. In cryogenic processing, the substrate is typically cooled by reducing the temperature of the platen upon which the wafer is disposed in the range of between room temperature to about −100° C. Lower wafer temperatures increase the amount of amorphization caused when ions from a ribbon beam in an ion implanter hit the substrate (damage layer) thereby providing better process control.
One way to cool wafers for cryogenic processing is to use an integrated cooling system operative with ion implantation system which may typically utilize a closed loop Nitrogen gas that removes heat from the wafer process platen. Such gas cooling systems are complex and expensive due to the need for external low temperature chillers, gas compressors, and associated power systems, as well as vacuum insulated cryogenic plumbing outside and inside the vacuum chamber. It would reduce cost, system complexity, and increase reliability, if instead a thermosyphon system could be used to remove heat from the wafer process platen and/or precooling platens, thereby obviating the need for some of the prior complex systems.
Generally, a thermosyphon is a system that transfers heat via natural convection in a fluid. The natural convection is driven by gravity with the colder, denser fluid flowing downward toward a location where heat is to be removed and the warmer, less dense fluid flows upward to pulling the heat away from such a location. Thus, thermosyphon systems connect an object to be cooled with a reservoir or device providing the cooling. Thermosyphon systems have a number of advantages. They are passive devices requiring no external pumping to provide fluid flow and heat transfer. This leads to simpler, more reliable systems. Since the thermal conductivity of most materials at cryogenic temperatures is quite low, thermosyphons can in many cases transfer heat more efficiently than solid conduction.